One proposed technique adopted for an internal combustion engine system applies a high pass filter to detect the occurrence of an engine misfire in the condition of low engine rotation speed, while applying a low pass filter to detect the occurrence of an engine misfire in the condition of high engine rotation speed (see, for example, Patent Document 1) In the condition of the low engine rotation speed, there is a low frequency noise caused by an ‘engine swing back’ in the process of detecting a rotational variation of a crankshaft. The high pass filter is thus applied for engine misfire detection in the condition of the low engine rotation speed to eliminate the low frequency noise. In the condition of the high engine rotation speed, on the other hand, there is a high frequency noise caused by a distortional vibration or ‘rattling’ of the crankshaft in the process of detecting the rotational variation of the crankshaft. The low pass filter is thus applied for engine misfire detection in the condition of the high engine rotation speed to eliminate the high frequency noise. The internal combustion engine system of this prior art structure is expected to detect the occurrence of an engine misfire with high accuracy in any range of the rotation speed of the engine.
Patent Document 1: Japanese Patent Laid-Open No. H05-180064